Accurate velocity estimation for submerged vessels, such as submarines, is very important for some applications. For example, accurate velocity estimation improves the accuracy of certain on-board missile-delivery systems.
Correlation SONAR is capable of providing the most accurate measure of absolute velocity for a submerged vessel. A correlation SONAR system typically includes a SONAR source (projector), a receiver array (of hydrophones), and signal processing hardware and software. The position of each receiver within the array is fixed and known to a high degree of accuracy. The projector(s) directs a series of acoustic pulses towards the ocean floor and the receivers detect echoes of those pulses.
As discussed further below, correlation SONARS rely on selecting a best or maximum “correlation” either between hydrophones or pulses, for the determination of velocity. Maximum correlation occurs when the ray path of an initial SONAR transmission (from the transmitter to the ocean floor, etc., and back to a receiver) of a first detected pulse is equal to the ray path of a second SONAR transmission.
There are two types of correlation SONAR: spatial and temporal. Spatial-correlation SONAR estimates the velocity of a vessel by transmitting two or more pulses towards the ocean bottom, detecting echoes of the pulses on a planar two-dimensional array of hydrophones, determining which two hydrophones in the array correlate the best, and dividing the distance between those hydrophones by twice the time differential between the pulses. The time differential between the detected pulses for which maximum correlation occurs is referred to as the “optimal-correlation time”, CTo. In some cases, no two hydrophones will have a spacing that results in a maximum correlation. For example, peak correlation may occur between two hydrophone locations. In this case, an interpolation scheme is used as a part of the velocity estimation. Interpolation, however, reduces the accuracy of the velocity estimate.
Temporal-correlation SONAR also estimates the velocity of a vessel by transmitting two or more pulses toward the ocean bottom and detecting echoes of the pulses at a hydrophone array. For a given pair of hydrophones, the temporal system determines which two pulses correlate the best, and calculates velocity by dividing the fixed distance between the hydrophones by twice the time differential between the two correlated pulses.
Correlation SONARs provides an estimate of velocity for discrete times corresponding to when the pulses were sent and received. To provide a continuous estimate of velocity, an inertial system (e.g., gyroscopes, etc.) is typically used. But a velocity estimate obtained from an inertial system is known to be far less accurate than those obtained by correlation SONAR (due to gyroscope drift, etc.) on a long term basis. A Correlation SONAR yields relatively noisy pulse pair velocity estimates but has little bias error. In contrast, an inertial system is very accurate in a high frequency sense but is characterized by long term errors.
To provide a continuous estimate of velocity that is more accurate than can be obtained via an inertial system, the velocity estimate from a correlation SONAR system is used to “correct” the velocity estimate obtained via the inertial system. More particularly, a velocity estimate from the inertial system and a velocity estimate from the correlation SONAR system are obtained at the same time. The difference between those two estimates is calculated and filtered over time and the result is used to correct the continuous estimates of velocity from the inertial system. This filtering process reduces the high frequency noise error in the SONAR velocity estimates while retaining the benefit of the low bias error SONAR data. The correction factor is recalculated on a frequent basis to provide a current correction to inertial-system-based velocity estimates.
For applications in which an absolute (i.e., ground-referenced) velocity estimate is required, such as for a missile launch system, the acoustic pulses from the SONAR system projector must be directed toward a stationary feature. In the ocean, that feature is the ocean bottom. Unfortunately, correlation SONAR, like other types of SONAR, is subject to performance degradation when it is operated over irregular or otherwise problematic ocean-bottom terrain. The problem arises because such terrain affects the bottom return in a variety of ways that are problematic for existing SONAR processing techniques. Examples of problematic terrain includes highly sloped regions, regions that are particularly rough or reflective, seamounts, fracture zones, ridges, the continental shelf, and escarpments.
It would be useful to identify the error mechanisms that cause the performance degradation of correlation SONAR systems. Once identified, it would be beneficial to develop improved correlation SONAR processing methods to mitigate, to the extent possible, the performance degradation that otherwise results.